This is a U.S. National Phase Application Under 35 USC 371 and applicant herewith claims the benefit of priority of PCT/IT00/00087 filed Mar. 17, 2000, which was published Under PCT Article 21(2) in English and Application No. TO99A000241 filed in Italy on Mar. 29, 1999.
The system according to the invention is designed to obtain the operating alignment between two or more printheads containing different coloured inks, mounted on the scanning carnage of an ink jet dot matrix printer.
Ink jet colour printers are widely known, both thermal type and piezoelectric type, provided with a multiplicity of monochromatic heads (typically three or four) containing different coloured inks (typically corresponding to the fundamental colours cyan, yellow and magenta, with sometimes black); each head possesses a large number of nozzles for the ejection of the droplets of ink (for example three hundred, but the current technological trend is leading to even greater numbers) arranged at a constant pitch in one or more parallel rows, with a like number of ejecting elements for generating the droplets of ink selectively ejected through the nozzles corresponding to each one.
As is known in the most recent art, the thermal type ink jet printheads comprise a substrate or xe2x80x9cchipxe2x80x9d of semiconductor material (generally Silicon) on which the ejection resistors and the power drivers with which to drive them and also the logic for selection of the single ejection resistor to be driven are made, using known technologies; for the first-named, thin film technology is normally used, for the second, LDMOS technology (xe2x80x9clateral double diffused MOSxe2x80x9d) and for the third, CMOS technology.
The precision of relative positioning of the nozzles among each other on a single head is very high, since the nozzle carrier plate is made all of a piece and the active part of the head is produced on a single silicon chip, using microlithic-photographic techniques guaranteeing considerable mechanical precision. Not so high is the positioning precision with which the chip is assembled on the body of the container of the head. The head, in turn, is mounted on the scanning carriage of the printer, so that the final alignment of the nozzles among the various monochromatic heads (needed to produce good quality printing, especially in high definition, as is known to those acquainted with the sector art) can only be obtained by means of additional operative head aligning operations to be effected, more or less automatically, directly on the printer, with resultant difficulties of a practical and economic nature.
Various methods have been proposed for automating the alignment of the different monochromatic heads, such as for example those described in U.S. Pat. Nos. 5,644,344, 5,600,350, 5,451,990, 5,448,269, 5,404,020, 5,289,208, 5,250,956 and EP 0 674 993. In all these cases, a specimen plot is printed on a sheet and the positional errors of the sheet are subsequently detected.
Another class of solutions, such as for example those described in U.S. Pat. Nos. 5,499,098, 5,350,929, 5,276,467 and EP 0 734 877, comprises the use of masks or grids through which the misalignments between the heads are detected by means of optical devices.
The U.S. Pat. No. 4,709,248 presents a device consisting of an illuminating device, an optical detector capable of picking up a known characteristic of the heads and a linear encoder by means of which to measure precisely the position of the print carriage along the direction of its travel. The misalignments between the heads are obtained from the measurement of the carriage""s position whereas the optical system picks up transit of the known characteristic of each single head.
In the Italian patent No. IT 1 294 891, corresponding to the International patent application No. PCT/IT98/00251, an aligning method for multiple ink jet colour printheads is presented, together with a relative printhead and built-in optical position detector, though practical production difficulties exist due to the non-linearity of the electro-optical position sensors.
The object of this invention is that of defining a system for obtaining the operative alignment, both horizontal (scanning direction) and vertical (line feed direction), of the printheads of an ink jet colour printer provided with multiple monochromatic heads, with the precision and linearity necessary for high quality colour printing at high definition
The system of the invention is based on the availability of printheads comprising at least one optoelectronic device which acts as an optical position sensor, made of a column of phototransistors, built into the same chip as the head, i.e. made in the course of the same production process, with the same process steps and the same masks as are needed in any case to produce an integrated thermal ink jet head, and therefore without any increase of the costs and difficulty with respect to the known heads.
In this way, the integrated optoelectronic device acts as an optical position sensor aligned with the nozzles with photolithographic precision, with which it is possible to detect automatically, via the procedure described below, both the horizontal and the vertical position of each single monochromatic head mounted on the scanning carriage; the system of the invention uses the position readings thus made to effect, through the printer""s electronic controller, the appropriate corrections with which to compensate the geometric alignment errors encountered.
The horizontal alignment errors are corrected by appropriately delaying or advancing the ejection of the droplets of ink by the various monochromatic printheads in relation to the difference between the theoretical position and the real position of the head itself; the vertical alignment errors, on the other hand, are corrected by suitably staggering electronic driving of the nozzles by one or more positions, accepting a maximum misalignment equal to one half of the pitch between the nozzles and not using the nozzles of each head located outside a common alignment band.
Another object of this invention is that of defining a rapid and precise method for aligning the nozzle carrier plate with respect to the silicon substrate during the head manufacturing process, avoiding critical factors due to variations of the optical contrast between different batches of film found in other methods, using viewing systems for the alignment.
A further object of the invention is that of defining a rapid and precise method for aligning the subassembly consisting of the nozzle carrier plate and the silicon substrate, on the plastic body of the head.
The above-mentioned objects are obtained by means of an aligning method for multiple ink jet colour printheads with built-in optoelectronic position detector, characterized as defined in the main claims.
These and other objects, characteristics and advantages of the invention will be apparent from the description that follows of a preferred embodiment, provided purely by way of an illustrative, non-restrictive example, and with reference to the accompanying drawings.